1. Field of the Invention
The invention relates to a method for the simultaneous measurement of three velocity components with the aid of laser Doppler anemometry using two continuous-wave lasers for the production of three interference fringe systems of different colours in a probe volume. This method is particularly used for measuring all three components of the velocity of a fluid flow.
2. Description of the Prior Art
In many flow studies the simultaneous measurement of all three components of the fluid flow velocity is of interest. For the majority of fluid flows, the shear stress is for instance one of the most important characteristics, which can only be determined by simultaneously measuring all three components of the velocity vector. In laser Doppler anemometry, in which small particles are used as flow velocity indicators, this means that either all velocity components of each individual particle have to be recorded or that three components of different particles which pass within a short time interval have to be acquired. The admissible duration of the time interval is in this case dependent on the time scale of the turbulence in the flow. For supersonic flows, for instance, the duration of this time interval must not exceed a few microseconds.
Laser Doppler anemometry, which is the basis of the invention described herein, is occasionally also termed "Doppler-difference method" or "fringe anemometry" (B. Lehmann, H. J. Pfeifer, H. D. vom Stein, DE-OS No. 16 73 403).
The reason for this is that two partial beams emanating from a laser are simultaneously directed onto the moving object the velocity of which has to be measured. Because of the difference in the angles, the two beams undergo slightly different Doppler shifts in the scattering process and this difference is used as a measure of the velocity. An equivalent explanation starts from the fact that the two partial beams generate an interference fringe pattern in the intersection volume and as an object passes through this fringe system, it appears alternately bright and dark and the modulation frequency of the light scattered in this way from the object is proportional to the velocity component in the plane perpendicular to the interference fringe system. In the simplest case such a fringe anemometer can therefore detect only a single velocity component.
In the past several optical arrangements have been desribed, which allow three velocity components to be determined simultaneously with the aid of laser Doppler anemometry. They are all extensions of two-component systems working with two colours of an Argon-Ion laser. With these two-component techniques the two strong lines emitted at the wavelengths 488 nm and 515 nm are filtered out and used to generate two interference fringe patterns in the probe volume, the fringes of different colours thereby being normal to each other at a large angle, in general 90 degrees. For both the production of the interference fringe system and the recording of the light scattered, the filtering process is either performed by dichroic reflectors or by interference filters and dispersion prisms.
Existing three-component systems may be divided into various categories. In the first one, simply only a third wavelength of the Argon-Ion laser is used to produce a third interference fringe system, the orientation of which is at a high angle with respect to the other two (W. J. YANTA, A Three Dimensional Laser Doppler Velocimeter for Use in Wind Tunnels, ICIASF"79 Record, IEEE Publication 79 CH 1500-8 AE, pp. 294-301, 1979). This method suffers from two drawbacks. On the one hand the output power of the third strongest line of the Argon laser is strongly below that of the two aforementioned lines. On the other hand this line at a wavelength of 477 nm is separated from the line at 488 nm by 11 nm only. Dichroic elements are no longer capable of separating the two lines. Interference filters are in the present case subject to strong losses and dispersion optics need long optical ways in the case of such a small line spacing, causing thereby instabilities in the whole arrangement. The same statements are true for the 497 nm line which is nearly as strong.
The second device for the simultaneous measurement of three components uses only the two strongest lines of the Argon-Ion lasr. In addition to the four beams of the two-component system a fifth laser beam either at 488 nm or at 515 nm is directed into the probe volume ("LDV System 9100-11 for Three Component Measurement", paper published by the TSI firm, Inc. 500 Cardington Road, St. Paul, Minn. 55164 USA). This partial beam produces a third fringe system along the propagation direction of the laser beams. The separation of the signals is in the present case provided by electronic means. A substantial disadvantage of this device is that for physical reasons in one interference fringe pattern the fringe spacing must be extremely small, i.e. of the order of 1 to 2 microns. Otherwise the fringe spacing becomes too large in the third system. Due to this small fringe spacing, this technique is only suited for very low velocities and it can never be used for velocities above 100 m/s.
Also in a third existing system for three-component measurements two colours of an Argon-Ion laser are used. With one of these colours, two orthogonal fringe systems are generated and separated by polarization. The third component is measured by a fringe system made of the second colour ("Laser Doppler Anemometry", pages 44 and 45, paper published by the DISA Elektronik A/S, Mileparken 22, 2740 Skovlunde, Denmark). The separation of two components by means of polarization, however, is only applicable if the particles do not change the polarization direction in the scattering process. However, this is very often the case so that the risk of an interaction between the two measurements cannot be excluded.
A similar set-up also uses one colour to determine two components. In this case two Bragg cells, operating at two different frequencies, provide at the same time the splitting of the beam into four partial beams and the different displacement speeds of the interference fringes generated in the probe volume and displayed nearly orthogonally to each other (F. L. Eltsley, F. L. Crosswy and D. Brayton, Transonic Wing/Store Flow Field Measurement Using a Laser Velocimeter, Technical Report AEDC-TR-80-54 1980, Arnold Engineering Development Center, Arnold Air Force Station, Tenn. 37389, USA). Separation of the two velocity components is carried out by electronic means again. The third component is recorded by the second colour again. In flows of high turbulence intensity, which are of major interest to the investigations conducted in fluid mechanics, separation of the two aforementioned signals is difficult and in many cases not possible.
Another technique should be mentioned here which allows the third component to be measured by the direct Doppler effect. The light scattered back from the particles is in part superposed to the initial beam of light emanating from the laser. This leads to a different in the frequencies, which allows the velocity component to be measured directly along the propagation direction of the laser beams. This method works only in the back scattering mode and is therefore limited to low velocities. With this method only large-size inert particles can be detected.
The German Pat. No. 31 06 025 (B. Lehmann) indicates that it is also possible to detect at the same time three velocity components with the aid of the direct Doppler technique according to Smeets. In this case essential characteristics of the fringe type anemometer are lost, for instance the possibility of observing single particles, working under the favorable conditions offered by the forward scattering mode, and using simple lasers presenting a short coherence length.
Finally a method should be mentioned in which two Argon-Ion lasers are used. In this case, one of the lasers produces the usual two-colour system at wavelengths of 488 nm and 515 nm. The second laser generates an interference fringe system at 477 nm (A. Boutier, "Three Dimensional Laser Velocimetry: A Review", Proc. Second Intern. Symposium on Applications of Laser Anemometry to Fluid Mechanics, paper No. 10.5, 1984, Instituto Superior Tecnico, Mech. Engn. Dept., 1096 Lisboa codex, Portugal). This technique differs from the aforementioned methods in that a relatively weak laser is used for the two-colour system whereas a strong laser is put into operation for the third colour. However, the aforementioned disadvantages inherent is this three-colour system, and pertaining to wavelengths which are too close together cannot be eliminated. In addition arrangements of this type including an Argon laser are obviously quite expensive. The only advantage over the first method mentioned above is that the amplitude of the scattered light is nearly same for all the three colours.